This is a proposal to determine whether antagonistic interactions between polarity complex proteins prevent the abnormal Yap/Taz mitogenic signals that underlie cortical malformations. Stem cell like apical neural progenitor cells (aNPCs) form a pseudostratified neuroepithelium maintained by apical junctions comprised of adherens and tight junction components and actomyosin cables. Disruption of the neuroepithelium causes abnormalities in neuronal differentiation and migration, and leads to cortical malformations in a spectrum of neurodevelopmental disorders, such as periventricular heterotopia (PH), microcephaly and ventriculomegaly. In the neuroepithelium, two protein complexes (Crb/Pals1/Patj or Mupp1 and Par3/Par6/aPKC), termed ?apical polarity complex proteins,? are located on the apical side, and one basal polarity complex (Lgl/DLG/Scb) is located basolaterally. This asymmetrical distribution is critical for establishing cell polarity and determining the asymmetrical fates of the aNPCs, but how these apical and basal protein complexes interact is not known, nor whether such interaction is required for correct cortical epithelial structure and aNPC division. To unravel this problem, we have generated a cortex-specific Llgl1 mutant characterized by massive PH, and a cortex-specific Crb2 mutant with ventriculomegaly. Remarkably, Crb2 loss drastically reduces heterotopia in the Llg1 mutant in a genetic dosage dependent manner. It was shown previously that Llgl binds to myosin and regulate activity, whereas apical polarity complex proteins aPKC and Crb2 can inhibit Myosin activity. As Myosin activity is critical in establishment of actomyosin cable at the junction, it is plausible that its balanced regulation by polarity complex proteins is required for junctional integrity. Because cellular tension and junction defects are associated with mitogenic signals, such as the transcription coactivators Yap/Taz (downstream effectors of the Hippo pathway), the absence of Lgl1 function in Myosin regulation may cause the abnormal proliferation that produces the enlarged heterotopic cortex found in our Llgl1 CKO. Remarkably, deletion of Yap/Taz in Llgl1 mutants strikingly diminished heterotopia and restored hippocampal lamination. We hypothesize that the antagonistic regulation of Myosin activity by polarity complex proteins is required to establish/localize actomyosin at the apical junction and prevent activation of an abnormal Yap/Taz-dependent mitogenic signal. To test this, we propose to determine whether apical and basal complexes have opposite effects on cortical progenitor proliferation and junctional integrity. Next, we will determine how polarity complex proteins regulate junctional integrity to test whether Myosin activity is regulated by apical and basal polarity complexes that establishes/localize apical junctions. Finally, we will determine how Yap-Taz is abnormally regulated after polarity disruption to establish Yap/Taz as a critical mediator of abnormal proliferation due to the inhibition of upstream regulators when cellular junction is disrupted. The insights provided by this study into the pathogenesis of cortical malformations will provide a basis for novel treatments to prevent cortex malformation.